A solution polymerization process in which polymerization is conducted in a liquid phase homogeneous system, a slurry polymerization process in which polymerization is conducted in a solid-liquid heterogeneous system, and a gas phase polymerization process in which polymerization is conducted in a gas-solid heterogeneous system have been hitherto known as processes for producing polyolefins. Of these processes, the solution polymerization process (homogeneous polymerization process) is known to provide polyolefins of high quality, but this process is not always excellent economically because the size of the reactor tends to be enlarged or a large-scale recovery and purification equipment for solvent is necessary.
On the other hand, in the heterogeneous polymerization process such as the slurry polymerization process or the gas phase polymerization process, the recovery and purification equipment for solvent is unnecessary or it can be made extremely small scale, so that polyolefins can be produced simply and efficiently, resulting in economical advantages.
In the polymerization of olefins, transition metal compounds containing transition metals such as titanium and zirconium are generally employed as catalyst components. However, if the catalyst components are used as they are in the heterogeneous polymerization system, string-like polyolefin may be produced or the resulting polyolefin may adhere to a wall surface of the reactor to cause unfeasibility of operations. In such cases, it becomes difficult to perform stable polymerization.
These problems can be solved by previously forming a solid catalyst wherein the catalyst components are supported on a particulate carrier, such as silica, having a particle diameter of not less than a certain value (generally about several tens .mu.m) and using the catalyst for the reaction. However, when the catalyst components are supported on the particulate carrier, the catalytic effectiveness is sometimes decreased to lower the catalytic activity.
In the heterogeneous polymerization using the solid catalyst, the reactive monomer (olefin) penetrates and diffuses into the solid polymer containing the polymerization catalyst components and undergoes reaction. In some cases, however, the monomer diffuses ununiformly toward the active sites of the polymerization catalyst in the solid polymer to cause ununiform distribution of monomer concentration. As a result, it becomes difficult to obtain polyolefins of uniform properties, and the resulting polyolefins are generally inferior in the quality to those obtained by the homogeneous polymerization.
As for the polymerization solid catalyst for use in the heterogeneous polymerization, it is known that polyolefin of uniform particle property is obtained if a polymerization catalyst of uniform particle property is used.
In the heterogeneous polymerization, therefore, it is desired to easily obtain solid catalysts having small particle diameters and excellent particle property. By the use of such solid catalysts in the heterogeneous polymerization, the polymerization can be stably achieved, and besides polyolefins of high quality and uniform particle property can be obtained. Therefore, development of such solid catalysts is of industrially great value.
Under such circumstances as mentioned above, the present inventors have earnestly studied olefin polymerization solid catalysts. As a result, they have found that, when an olefin is prepolymerized onto an olefin polymerization catalyst in a heterogeneous system under irradiation with an elastic wave, a high-performance prepolymerized solid catalyst can be easily obtained without adhesion of polyolefin to the wall surface of a reactor in the prepolymerization process or without production of string-like polyolefin. The present inventors have further found that use of the prepolymerized solid catalyst in a slurry polymerization process or a gas phase polymerization process makes it possible to perform stable polymerization and to obtain polyolefin of uniform property and high quality. Based on the above finding, the present invention has been accomplished.